Gas and particle sensor using voltage and current behavior between electrodes

ABSTRACT

A sensor for detecting properties of a gas, gas mixture, or a gas or gas mixture containing particles, all collectively referred to as a “gas”. A flow tube contains a pair of electrodes arranged such that at least a portion of the gas flows between the electrodes. A controller applies voltage to the electrodes and measures response data from the electrodes representing the voltage-current relationship and voltage breakdown between the electrodes while the gas is between the electrodes. Based on the response data, the controller determine a concentration of the gas or a concentration of particles within the gas.

TECHNICAL FIELD OF THE INVENTION

This invention relates to sensors for gasses and gas mixtures andparticles in gasses or mixtures, and more particularly to such sensorsusing electrical characteristics between two electrodes.

BACKGROUND OF THE INVENTION

Paschen's law is an equation that gives the breakdown voltage, that is,the voltage necessary to start a discharge or electric arc, between twoelectrodes in a gas as a function of pressure and gap length. With aconstant gap length, the voltage necessary to arc across the gapdecreases as pressure is increased and then increases gradually,exceeding its original value. With constant pressure, the voltage neededto cause an arc decreases as the gap size is reduced but only to apoint. As the gap is reduced further, micro-scale physical laws dominateand micro-discharges due to field emission phenomena become an importantfactor.

For a given gas, the voltage between electrodes is a function of theproduct of the pressure and the gap distance. The curve of voltageversus the pressure-gap product is called Paschen's curve. Paschen's lawis the equation that fits these curves.

At higher pressures and gap lengths, the breakdown voltage isapproximately proportional to the product of pressure and gap length.However, this is only roughly true, over a limited range of the curve.

Various gas and particle sensors have been developed that make use ofPaschen's law.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 illustrates the sensor of the invention schematically andillustrates its principle of operation.

FIG. 2 illustrates one embodiment of the sensor of FIG. 1.

FIG. 3 illustrates how voltage breakdown values can be used to determinethe type and concentration of a gas.

FIGS. 4A and 4B illustrate how the voltage-current relationship prior tovoltage breakdown may be used to determine particle type andconcentration in a gas.

FIG. 5 illustrates a transition region to voltage breakdown which can beused to determine gas concentration and particle concentration.

FIG. 6 illustrates a gas/particle sensor installed in a vehicle, used totest automotive emission.

FIG. 7 illustrates the gas/particle sensor of FIG. 6 in further detail.

DETAILED DESCRIPTION OF THE INVENTION

The following description is directed to sensing gases and particles andtheir mixtures. The sensing has application to gases, gas mixtures, andto gases or gas mixtures containing particles, which may be allcollectively referred herein to as a “gas” or “gases”.

The sensing is performed by exploiting the voltage breakdown and voltageand current behavior between two electrodes. The presence of a gas orgas mixture (with or without particles) results in a response signalthat indicates gas type(s), gas concentration(s), and particleconcentration if particles are present.

The sensor has many applications and is especially suitable forinstallation in a vehicle to sense the properties of engine exhaust. Thesensor is robust and accurate. It can be made as small as a conventionalspark plug, meeting size requirements for current and future vehiclesusing internal combustion engines and onboard diagnostics.

Gas/Particle Sensor

FIG. 1 schematically illustrates a gas/particle sensor 100 in accordancewith the invention. A gas of interest is flowing through a pipe, tube,or other structure 101. For vehicle exhaust applications, the gas isflowing through an exhaust line.

In the example of FIG. 1, the gas contains particles. The flowing gasalters the dielectric properties of the medium between two electrodes102 to which voltage is applied. Sensor 100 exploits voltage breakdownand voltage-current properties to infer information about the type,composition, and properties of the medium between the two electrodes102. At certain voltages depending on the composition of the gas, avoltage breakdown region may occur between electrodes 102.

A motor 110 is mechanically connected to one of the electrodes 102 andis operable to move that electrode 102 toward or away from the otherelectrode. This allows the gap distance between electrodes to be varied.Changing the gap distance changes the electric field across the gap.Where E is the electric field, V is the applied voltage, and d is thegap distance: E=V/d.

The ability to change the gap distance is a feature of sensor 100 thataccommodates different particle size applications. Larger particles mayrequire a larger gap distance. It is expected that for variousapplications, sensor 100 may accommodate particles in a range of 5nanometers to 500,000 nanometers in diameter.

A processor 120 comprises appropriate equipment to electrically activateelectrodes 102 and to receive and measure response signals representingresponse current and voltage. As explained below, processor 120 alsoreceives and processes measurements from temperature, flow, and pressuresensors (not shown).

As explained below, the unique signatures of the voltage breakdown andvoltage-current relationships are functions of the particleconcentration, gas type and concentration, flow rate, and ambientmeasurements such as temperature and humidity.

With regard to gas composition, different gases will exhibit differentPaschen curves and voltage-current relationships. For example, in engineexhaust, the voltage breakdown and the voltage-current curves mayexhibit different signatures for CO₂, H₂O, O₂, N₂ and other gases thatare typically present in engine exhaust. While an important applicationof the sensor is for particle characterization in engine exhaust, thesensor can be used for gas species detection in any gas.

FIG. 2 illustrates one embodiment of gas/particle sensor 100, agas/particle sensor 200. The sensor 200 is robust, with an expectedtemperature range of 20 degrees C. to +500 degrees C.

A flow tube 201 has a shape and size suitable for flow of gas withinflow tube 201 from an inlet end to an outlet end. For automotive exhaustsensing applications, sensor 200 can be made very small, such asspark-plug size. For other applications, the size may vary depending onthe application.

An example of a suitable material and geometry for flow tube 201 is analuminum tube casing with funnel-like entry and exit openings. Thisconic design optimizes a consistent gas flow within the flow tube.

An example of suitable dimensions for flow tube 201 are a tube length of10 centimeters and tube diameter of 2 centimeters. These dimensionsdefine the overall physical size of sensor 200.

A pair of high voltage electrodes 202 are placed within flow tube 201,arranged so that at least a portion of the flow passes between them.These electrodes 202 form a micro-discharge chamber within the flow tube201. Depending on the application of sensor 200, an expected electrodesize is an effective area diameter in the range of about 2 micrometersto 100 millimeters.

In the embodiment of FIG. 2, electrodes 202 are parallel-plate planarelectrodes. However, in other embodiments, electrodes of various shapescould be used, such as pointed-to-planar or pointed electrodes.

In other embodiments, more than one pair of electrodes may be used inseries. This configuration could be used to resolve particle type andconcentration simultaneously for particular gases and gas mixtures.

Motor 203 is operable to change the distance between electrodes 201. Anexample of a suitable motor 203 is a motorized differential micrometer.

Sensors for measuring temperature, flow speed, and pressure within flowtube 201 are located in flow tube 201. These sensors are identified assensors 204 a, 204 b, and 204 c, respectively.

Controller 208 supplies an applied voltage to be applied to theelectrodes 202. The applied voltage may be varied. A typical sensor 200could have an applied voltage of up to 1 kV or higher depending on theelectrode gap size chosen.

As stated above, controller 208 measures the current and voltage betweenelectrodes 202. Other functions of controller 208 are to control motor203 and to receive measurement data from sensors 204 a, 204 b, and 204c.

Controller 208 is assumed to have appropriate hardware and software forperforming the methods and tasks described herein. Controller 208 may beimplemented as a dedicated electronic circuit board.

In operation, a gas or gas mixture (which may or may not containparticles) enters sensor 200 through the inlet and exits via the outletof flow tube 201, sweeping by the electrodes 202 inside the flow tube. ADC voltage is applied to the electrodes 202 as the gas is between them(still or flowing), and the voltage and current are measured. Controlunit 208 processes measurement data from sensors 204 a, 204 b and 204 c,as well as the voltage and current data (herein referred to as “responsedata”) to obtain characteristics of the gas or gas mixture as explainedbelow.

For a gas or gas mixture that contains particles, motor 203 can beactuated to change the gap distance. This maintains a desired particlesize to gap distance ratio as described above.

As explained below, sensor 200 provides real time information of thegas-particle mixture content by exploiting the voltage breakdown (VB)and voltage-current (VI) properties in that medium. If particles arepresent, information about the size and composition can also beinferred.

Methods of Detecting Gas or Gas Mixture Properties (with or withoutParticles)

Voltage breakdown between electrodes 202 depends on the electron meanfree path and the distance between the electrodes. The electron meanfree path is the average distance the electrons travel before making acollision with gas atoms and is related to the pressure and thetemperature.

If the distance between electrodes 202 and the temperature of the gasare fixed, the gas pressure may vary. At high pressure, the gas densityis higher and the electron free mean path is very short. The electronsthus have very little time to accelerate and gain energy before losingit to collisions, leading to a high VB value at high pressures. Incontrast, at low pressures, electrons are less likely to collide withgas atoms and they can indeed be accelerated to ionizing energies.However, at very low pressures, the probability of hitting gas atomsbecomes very low and most of them hit the anode directly, setting severelimits on the avalanche process. In this case, the voltage breakdownthreshold becomes very high.

Between the extremes of very high and very low pressures, there exists aregion where the breakdown occurs as a function of the gas properties,pressure, the electrode material, and separation. This is known as the“Paschen's minimum.” Different gases have different Paschen curves, andthis knowledge can be used to detect what species of gas is present inthe sensor.

The presence of particles in the gas can affect the breakdown threshold.This behavior is further affected by gap distance. For a particular gas,this knowledge can be used to determine the presence and concentrationof particles in that gas.

FIG. 3 is a plot of experimental data as an example of the voltagebreakdown response of sensor 200 as a function of changing gasconcentration. In the example of FIG. 3, the gas is CO2, but the sameconcept applies to other gases.

FIG. 3 illustrates how sensor 200 can be used to measure gas or particleconcentrations from voltage breakdown values. Voltage breakdown changesas a function of the medium between the electrodes.

Referring again to FIGS. 1 and 2, changing the gap distance betweenelectrodes would have a similar effect on the electric field as changingthe applied voltage. In other words, the results of FIG. 3 can beobtained by either changing V or by changing d via motor 203. Anadvantage of changing d is the accommodation of differently sizedparticles. It is expected that the best results will occur if the ratioof mean particle size to the gap distance is in a range of about 1/100to 1/1000 (or smaller) to avoid gap fouling.

FIGS. 4A and 4B illustrate how sensor 200 may be used to measure currentresponse as an indicator of gas or particle concentration. In theexample of FIGS. 4A and 4B, the gas is exhaust gas and contains sootparticles, but the same concept applies to other gases, gas mixtures,and particle-containing gases. In the range before voltage breakdown(VB), the voltage-current (VI) relationship changes as a function ofboth gas concentration and particle size.

FIGS. 4A and 4B plots experimental data, with alternating flows ofnitrogen (N2) with no soot (FIG. 4A) and soot at 5 mg/m³ (FIG. 4B)passed through electrodes 202. A voltage of 700 volts was applied to theelectrodes 202, with a 70 micrometer gap between them.

The plots of FIG. 4A correspond to flows of nitrogen with no soot, andthe plots of FIG. 4B correspond to flows with 5 mg/m³ of soot. The plotsof FIGS. 4A and 4B should be read as a series from left (FIG. 4A) toright (FIG. 4B) and downward in progression. At first (top plots),sensor 200 did not respond to the presence of soot, as it requires somesurface seeding of soot particles before it provides a response and theparticles starts jumping from one electrode to the next. After the firstexposure with no response, sensor 200 was again exposed to nitrogen,then to soot again (second plots). With the second exposure, sensor 200first read zero, then began to respond to soot concentration with a twoorder of magnitude increase in current. Then, sensor 200 was exposed tonitrogen again, took some time for the path to clear, then went down tozero with nitrogen FIG. $A, third plot). This process was repeated forfour flows each of nitrogen and soot.

In practice, the effect of “early” or “late” response data, as sootcollects, may be used to inform the condition of an exhaustaftertreatment device in a maintenance test. If sensor 200 is exposed toengine exhaust containing particles for a period of time, the exhaustaftertreatment device could either pass or fail particle emissionstesting, depending on the sensor response.

FIGS. 4A and 4B further illustrate how particle size may be inferred. Ifflows through the electrodes 202 have two different size distributions,the mean diameter of the particles will be different. The time it takesfor the flow to reach steady-state current infers the size distribution.For example, a mean diameter of one flow might take 5 minutes whereas amean diameter of another flow might take 2 minutes to reach the samesteady state current.

Transition Region to Voltage Breakdown

FIG. 5 illustrates the voltage-current (VI) relationship betweenelectrodes 202, with emphasis on the transition region to voltagebreakdown (VB). It is assumed that the distance between electrodes isfixed, and FIG. 5 is for an arbitrary gas. The exact shape of the VItrace depends on a number of factors including the gas type, pressure,temperature, humidity, electrode geometry, material, surface roughness,size, etc.

At low voltages, the current between the electrodes is extremely small.As the voltage between the electrodes increases, the current increasesslowly. This nano-amp scale current comes from the charge carriersproduced by background ionization. These carriers are swept out of thegap by the electric field between the electrodes that is created by theapplied voltage. Since the number of charge carriers created by thebackground radiation (cosmic radiation or other ionizing sources) isfinite, the electric current quickly saturates. The voltage can then beincreased with no increase in current. The ions and electrons are pulledtowards the electrodes through the gas molecules interacting with themas they go. Further increase of the voltage facilitates the ionizationprocess in the gas: electrons become more energetic and able to ionizegas molecules. At this point, the voltage-current characteristics begintapering off near the breakdown voltage and the glow discharge becomesvisible once the breakdown is reached.

Further increase of the voltage leads to a large electron density at thecathode, heating it to the point where electrons are emittedthermo-ionically. At this point, glow-to-arc transition occurs, and theprocess moves to thermal arcing, in which the plasma becomes mostlyionized. The earlier part of this VB process is also known as the coldbreakdown, avalanche breakdown, or the Townsend discharge.

In the transition to VB, the VI profile to reaching breakdown isdependent on gas concentration. If the gas contains particles, the VIprofile will depend on particle concentration. For exhaust gasapplications, this trace is dependent on soot concentration. Thesignature of this transition profile can be used to detect theseconcentrations.

Vehicle Applications

FIG. 6 illustrates a gas/particle sensor 60 installed for use in testingemissions of a vehicle. The sensor 60 is installed within the tailpipe61, downstream of an exhaust aftertreatment device 62.

FIG. 7 illustrates sensor 60, which is similar to sensor 200, but itselectrodes 72 need not be motorized. As explained above, the distancebetween electrodes 72 is fixed and has a gap distance that is optimizedfor the application, and in particular for the expected range ofparticle sizes. As with sensor 200, the electrodes are illustrated asparallel plate electrodes, but may have other configurations.

A controller 78 provides an applied voltage and receives responsevoltage data, via electrodes 72. It uses this data to determine V-I andvoltage breakdown measurements for a flow of emissions passing throughthe electrodes.

In operation, sensor 60 is used to test the composition of the engineexhaust. As described above, gas species and concentrations, andparticle concentrations can be determined. For example, controller 78may store data of the V-I and/or VB characteristics of emissions from aproperly functioning exhaust aftertreatment device, and compare thisstored reference data to the current data. In this manner, the functionof the exhaust aftertreatment device 62 can be tested.

What is claimed is:
 1. A sensor for detecting properties of a gas, gasmixture, or a gas or gas mixture containing particles, all collectivelyreferred to as a “gas”, comprising: a flow tube; a pair of electrodeswithin the flow tube, arranged such that at least a portion of the gasflows between the electrodes; and a controller operable to apply voltageto the electrodes and to measure response data from the electrodesrepresenting the current between the electrodes while the gas is betweenthe electrodes; wherein the controller is further operable to processthe response data to determine a concentration of particles within thegas based on a current response prior to voltage breakdown over time asparticles collect within the flow tube.
 2. The sensor of claim 1,further comprising a motor mechanically attached to one of theelectrodes and operable to move the position of that electrode such thatthe distance between the electrodes may be varied.
 3. The sensor ofclaim 1, further comprising at least one additional sensor within theflow tube for measuring one or more of the following properties of thegas: temperature, flow rate, and pressure.
 4. The sensor of claim 1,wherein the controller is operable to process the response data todetermine the concentration of at least one gas component of the gasbased on the voltage breakdown of the gas.
 5. The sensor of claim 1,wherein the controller processes the response data by comparing theresponse data to stored data representing a reference response.
 6. Thesensor of claim 1, wherein the distance between the electrodes is afixed distance representing an optimal distance for determining theconcentration of a selected range of particle sizes.
 7. The sensor ofclaim 1, wherein the electrodes are parallel plate electrodes.
 8. Thesensor of claim 1, wherein the controller is operable to process theresponse data to determine the mean diameter of particles within thegas.
 9. A method of detecting properties of a gas, gas mixture, or a gasor gas mixture containing particles, all collectively referred to as a“gas”, comprising: delivering the gas to a sensor comprising a flowtube, a pair of parallel plate electrodes within the flow tube, arrangedsuch that at least a portion of the gas flows between the electrodes;applying voltage to the electrodes; varying the voltage applied to theelectrodes; measuring response data from the electrodes representing thecurrent between the electrodes while the gas is between the electrodes;processing the response data to determine a concentration of particlesin the gas, based on a current response prior to voltage breakdown overtime as particles collect within the flow tube.
 10. The method of claim9, wherein the step of varying the electric field is performed byvarying the voltage applied to the electrodes.
 11. The method of claim9, wherein the step of varying the electric field between the electrodesis performed by varying the gap distance between the electrodes.
 12. Themethod of claim 9, wherein the processing step is performed to determinethe concentration of at least one gas component of the gas based on thevoltage breakdown of the gas.
 13. A vehicle emissions sensor fordetecting properties of a gas, gas mixture, or a gas or gas mixturecontaining particles, all collectively referred to as a “gas”, withinthe exhaust of a vehicle, comprising: a flow tube; a pair of electrodeswithin the flow tube, arranged such that at least a portion of the gasflows between the electrodes; and a controller operable to apply voltageto the electrodes and to measure response data from the electrodesrepresenting the current between the electrodes while the gas is betweenthe electrodes; wherein the controller is further operable to processthe response data to determine a concentration of particles within thegas, based on a current response prior to voltage breakdown over time asparticles collect within the flow tube.
 14. The sensor of claim 13,wherein the electrodes are a fixed distance apart, the distance beingcalculated to provide optimum response data for a selected range ofparticle sizes.
 15. The sensor of claim 13, wherein the controllerprocesses the response data by comparing the response data to storeddata representing a reference response.
 16. The sensor of claim 13,wherein the electrodes are parallel plate electrodes.